Mold for hot-forming a thermoplastic lens

ABSTRACT

The mold comprises two dies mounted in a sheath to slide along the axis of the sheath and each presenting a forming face facing the other. The two dies and the sheath are provided with intrinsic heat transfer means. A parison or preform of said thermoplastic material is placed between the two dies and the assembly is enclosed in the sheath; the dies and the sheath are heated by their intrinsic heat transfer means up to a forming temperature. The dies are moved towards each other to shape the material by plastic deformation until a predetermined relative position is reached corresponding to the thickness desired for the lens; the dies and the sheath are cooled by their intrinsic heat transfer means down to an unmolding temperature; and the finished lens is extracted from the mold.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of co-pending application Ser. No.10/015,587, filed on Dec. 17, 2001, the entire contents of which arehereby incorporated by reference.

The present invention relates to manufacturing thermoplastic lenses,such as ophthalmic lenses or instrument lenses, obtained by appropriatemolding (known as “forming”) of a plastics material, in particular athermoplastic resin, such as polymethyl methacrylate, polycarbonate,polycarbonate copolymer, polynorbornene polystyrene, cyclic polyolefinsand their copolymers, etc.

BACKGROUND OF THE INVENTION

When making lenses by a method of this kind, it is known to takeadvantage of the ability possessed by plastics materials to deform whenheated in order to impart the desired shape to said lenses. Startingfrom a mass of thermoplastic material of standard shape (commonlyreferred to as a “parison”) or from a special preform of a shape that isclose to the desired shape, this shaping is performed by forming underpressure between two dies that are pressed by means of a press againstthe material that has previously been heated to the plastic state. Thedies are generally made of stainless steel and each has a forming facepossessing suitable curvature(s), corresponding to the curvature(s) tobe given to the finished lens, and presenting optical polish, i.e.analogous to a mirror polish.

To obtain lenses possessing optical qualities suitable for theirfunction, various precautions must be taken during manufacture, inparticular in order to avoid irregular deformations or the presence ofinternal tensions, said deformations or tensions giving rise toanisotropy or to undesirable aberrations.

In this respect, special care is taken in making molding dies.Furthermore, it is often recommended to perform manufacture in twosuccessive stages: firstly a blank is made to dimensions that are closeto those of the desired lens, e.g. by injection molding, forming, ormachining, and then the blank is finished by hot-forming.

These precautions relating to the tooling and the method of operationmust also be associated with precautions relating to the heating of theplastics material and of the mold during forming. Ordinarily, heating isprovided by the dies themselves, which dies are arranged to receive aheating fluid or gas, or electrical resistances. The heat provided byhot fluid flow or by electrical resistances spreads throughout the massof the dies and is communicated by the walls thereof to the plasticsmaterial while it is being formed.

Heating in that way presents drawbacks in practice.

Firstly, it does not ensure that the plastics material is heated inregular and uniform manner as would be desirable to avoid internaltensions. Different portions of the dies and, as a result, of thematerial to be shaped, are brought only progressively and unevenly tothe temperature required for forming. This lack of uniformity in thetransmission of heat also occurs during cooling after forming.

This unequal heating and cooling at various points of the shapedmaterial causes the resulting optical lens to retain deformations andtensions that often make it unsuitable for its intended opticalpurposes.

Furthermore, in usual arrangements, the dies for receiving the heatingand cooling devices are fixed on the press, and it is in considerationof this arrangement that connections are provided with heating fluidfeeds or with electricity feeds. With installations of that kind, theoperations of installing and removing dies are lengthy and complicated.

Another method of heating that seeks to mitigate those drawbacksconsists in using external heater means that are not part of the molditself, and in particular not part of the dies. The mold containing thepreform or parison is thus heated separately from the press prior tobeing installed therein for the forming operation. Nevertheless, thefact of the mold being heated separately from the press by externalmeans, although it does indeed enable the dies to be free of anyhydraulic or electrical connection, nevertheless suffers from the majordrawback of requiring the mold to be handled in each cycle, therebyconsiderably slowing down production throughput. In addition, thetemperature of the mold cannot be controlled during forming and noprovision is made for cooling the mold after forming, so like heating,cooling can be envisaged only by using external means, thus requiringfurther handling.

In order to enable the mold to be taken from a heating station to theforming station (i.e. the press), the mold has a sheath covering thedies and within which the dies are slidably mounted. The function of thesheath is then to ensure that the mold constitutes a mechanical unit byproviding a mechanical connection between the two dies independently ofthe press.

OBJECTS AND SUMMARY OF THE INVENTION

The invention provides a mold for hot-forming a thermoplastic lens, themold comprising two dies mounted in a sheath to slide along the axis ofthe sheath, and each presenting a transverse forming face facing theother, in which mold the two dies and the sheath are provided withintrinsic and both-way heat transfer means (i.e. for heating and forcooling). For example, the heat transfer means can comprise an internalcircuit for circulating a heat-conveying fluid.

This combines two advantages. Firstly, encasing the dies in a sheathwhose temperature is self-modulated makes it possible to improve thequality and in particular the uniformity of the heating or cooling ofthe mold, and as a result of the thermoplastic material. The sheath thusperforms a thermal containment function optionally associated with itsfunction of providing the two dies with mechanical guidance relative toeach other. In addition, integrating both-way heat transfer means in themain portions of the mold, i.e. the dies and the sheath, makes itpossible to provide both heating and cooling of the mold in applicationof an optimized temperature regulation relationship, without it beingnecessary to move the mold in order to subject it to the action ofexternal heating and/or cooling means.

According to an advantageous characteristic of the invention, each ofthe dies comprises:

-   -   a base containing at least a fraction of the heat transfer means        of said die, and means for connecting them to a heat transfer        source; and    -   a removable insert fitted to the base and carrying the forming        face.

The insert is thus interchangeable and as a result can be selected froma pre-established set or even manufactured on demand as a function ofthe surface definition desired for the lens. The insert is quick andeasy to install in and remove from the mold, given that the connectionto the temperature regulation source takes place via the base andtherefore does not need to be removed.

In which case, it is advantageous for the base of each die to receiveall of the heat transfer means of said die, and for the insert having nosuch means to be temperature regulated solely by transferring heat toand from the base. Mounting the insert on the base therefore does notrequire any hydraulic or electrical connection, whether with thetemperature regulation source or with the base.

The invention also provides a method of hot-forming a thermoplastic lensby means of the above-defined mold, the method comprising the steps of:

-   -   placing a parison or preform of said thermoplastic material        between the two dies and enclosing the assembly in the sheath;    -   heating the dies and the sheath by their intrinsic heat transfer        means up to a forming temperature;    -   moving the dies towards each other to shape the material by        plastic deformation until a predetermined relative position is        reached corresponding to the thickness desired for the lens;    -   cooling the dies and the sheath by their intrinsic heat transfer        means down to an unmolding temperature; and    -   extracting the finished lens from the mold.

Preferably, the forming temperature is higher than or equal to thevitreous transition temperature of the thermoplastic material used, goodresults have been obtained more precisely with a forming temperatureexceeding the vitreous transition temperature by a difference lying inthe range 30° F. to 120° F., and preferably about 45° F.

Conversely, it is preferable for the unmolding temperature to besignificantly below the vitreous transition temperature. For example anunmolding temperature can be provided which presents with respect to thevitreous transition temperature a difference lying in the range 20° F.to 50° F., and preferably about 35° F. It is indeed interesting not tocool too much the mold, so as to reduce as far as possible the time ofeach cycle.

According to another advantageous characteristic of the invention, thesheath possesses an annular recess that is open to the inside and thatcommunicates with the interstitial gap defined by the forming faces ofthe two dies. This annular recess is designed to receive the excessplastics material which, after forming, constitutes peripheral flashprojecting sideways from the dies.

The interstitial gap defined by the forming faces of the two diesco-operates with the annular recess to form a hermetically closed insidevolume which is connected to the outside via at least one suctionchannel passing through the sheath and opening out, for example, intothe annular recess.

In a mold presenting this characteristic, it is advantageous to proceedas follows: before and/or simultaneously with movement of the diestowards each other for the purpose of shaping the material by deformingit plastically, said suction channel is used to establish a relativevacuum in the hermetically closed inside volume of the mold.

Best results are obtained with a relative vacuum of lower than 0.25 bar,and preferably lying in the range 0 to 0.1 bar.

According to another advantageous characteristic of the invention, themold has a trimming ring slidably mounted in the sheath and optionallyco-operating with a lip bordering the annular recess.

In a mold presenting this characteristic, it is advantageous to proceedas follows: after the material has been fully formed by moving the diestowards each other, the trimming ring is actuated to cut off theperipheral flash formed by surplus material projecting sideways from thedies.

According to yet another advantageous characteristic of the invention,the sheath has two portions mounted to slide relative to each otheralong the same sliding axis as the dies between firstly an openconfiguration giving direct access to the interstitial gap defined bythe forming faces of the two dies, in particular for the purpose ofinserting the perform or parison between the two dies and for removingthe finished lens after forming, and secondly a closed configuration inwhich the two portions are in contact via junction faces in a transversejoin plane.

The annular recess of the sheath is then advantageously in the form of agroove or setback, preferably with draft taper, in the junction face ofat least one of the two portions of the sheath.

BRIEF DESCRIPTION OF THE DRAWING

Other characteristics and advantages of the invention will appear onreading the following description of a particular embodiment given byway of non-limiting example.

Reference is made to the accompanying drawings, in which FIGS. 1 to 4are diagrammatic section views through a mold of the invention showingsuccessive steps in the forming method of the invention.

MORE DETAILED DESCRIPTION

With reference to the figures, a mold of the invention for hot-forming athermoplastic lens comprises two dies 1, 2 slidably mounted in a sheath3, and each presenting a forming face 4, 5 facing the other. The sheath3 has a central axis 6 defining the direction of relative slidingbetween the dies 1, 2.

More precisely, the sheath 3 comprises two portions 7, 8 mounted on anexternal support (not shown) e.g. associated with the frame of a press,so as to be slidable relative to each other along the axis 6 betweenfirstly an open configuration as shown in FIG. 1 giving direct access tothe interstitial gap defined by the forming faces 4, 5 of the two dies,and secondly a closed configuration as shown in FIGS. 2 to 4 in whichthe two portions 7, 8 are in contact via junction faces 9, 10 lying in atransverse join plane J.

Each of the two dies 1, 2 comprises a base 11, 12 and a removable insert13, 14 fitted to the base and carrying the forming face 4, 5.

The two dies 1, 2 and the sheath 3 are provided with intrinsic heattransfer means, which are both-way i.e. suitable both for heating themand for cooling them. By way of example, the heat transfer means can beimplemented, as shown, in the form of internal circuits for circulatinga heat-conveying fluid such as an oil. There can thus be seen helicalcircuits 15, 16 formed in the portions 7, 8 of the sheath 3 and spiralcircuits 19, 20 formed in the bases 11, 12 of the dies 1, 2.

The base 11, 12 of each die thus receives all of the heat transfer meansof said die: the insert 13, 14 has no such means and its temperature isregulated solely by heat transfer with the base.

The two portions 7, 8 of the sheath 3 and the bases 11, 12 of the diesare provided with couplings 17, 18 and 21, 22 enabling the circuits 15,16 and 19, 20 to be connected to a common external temperatureregulation source S or to a plurality of sources which are distinct andindependent so as to regulate the various portions of the moldindependently of one another. In particular, provision can be made toconnect firstly the circuits 15, 16 of the sheath 3 to a first sourcefor providing temperature regulation of the two portions 7, 8 of thesheath 3, and secondly the circuits 19, 20 of the bases of the dies 1, 2to a second source for providing temperature regulation of the dies,independently of the sheath 3.

The bottom portion 7 of the sheath 3 has an annular groove 23 formed asa setback in the junction face 9 of the bottom portion 7 of the sheath3. This groove 23 is adjacent to the inside face of the portion 7 of thesheath 3 and is internally bordered by a circular rim 24 forming aflash-trimming lip, as explained in greater detail below. The groove 23forms a small amount of draft taper.

When the sheath 3 is in its closed configuration with its two portionsmeeting via their junction faces 9, 10, the groove 23 is covered atleast in part by the junction face 10 of the top portion of the sheath 3so as to form an annular recess 23. Nevertheless, the top of the lip 24is set back a little from the join plane J so as to leave a narrow gapleaving the recess 23 open to the inside of the sheath and incommunication with the interstitial gap defined by the forming faces 4,5 of the two dies 1, 2 where the preform or parison P is placed.

The interstitial gap defined by the forming faces 4, 5 of the two dies1, 2 thus co-operates with the annular recess 23 to define a single andhermetically closed inside volume. A suction channel 25 is providedthrough the bottom portion 7 of the sheath 3 and extends transverselyfrom the annular recess 23 to the outside face of the portion 7 of thesheath so as to enable said inside volume to be connected to theoutside. A coupling 26 is provided on the outside face of the portion 7of the sheath to couple the channel 25 to a vacuum pump (not shown) soas to establish a relative vacuum in said inside volume.

The mold also has a flash-trimming ring 30 slidably mounted in the topportion 8 of the sheath 3. More precisely, in the example shown, thering 30 is in the form of a portion of a tube about the axis 6 and isinterposed between the top die 2 and the top portion 8 of the sheath,with very small transverse clearance so as to provide accurate guidancefor the top die 2, the ring 30, and the top portion 8 of the sheathrelative to one another along the axis 6. The flash-trimming function ofthe ring 30 is explained in greater detail below.

To hot-form a thermoplastic lens by means of the mold as describedabove, the procedure is as follows.

A parison or preform P is placed between the two dies 1, 2. To do this,the starting configuration is as shown in FIG. 1, in which firstly thedies 1, 2 are far enough apart to enable the preform or parison P to beinserted between their forming faces 4, 5, and secondly the two portions7, 8 of the sheath 3 are spaced apart from each other in an openconfiguration so as to enable the preform or parison P to be inserteddirectly between the dies without any need to separate the dies 1, 2from the sheath 3.

The parison or preform P is made of a thermoplastic material such aspolymethyl methacrylate, polycarbonate, polycarbonate copolymer,polynorbornene, polystyrene, cyclic polyolefins and their copolymers,etc.

The two portions 7, 8 of the sheath are then closed against each other,meeting via their junction faces 9, 10, as shown in FIG. 2 so as to forma single sheath 3. The sheath 3 thus completely surrounds both dies.

The dies 1, 2 and the sheath 3 are then heated or pre-heated by theirintrinsic heat transfer means up to forming temperature. For thispurpose, in the example shown, the heat-conveying fluid is heated by theexternal temperature regulation source(s) S and is caused to circulatein the circuits 15, 16, 19, 20.

The forming temperature is preferably higher than or equal to thevitreous transition temperature of the thermoplastic material used. Goodresults have been obtained in particular with a forming temperature thatexceeds the vitreous transition temperature by an amount lying in therange 30° F. to 120° F., and preferably about 45° F.

It will be observed that during this heating, because of the presence ofthe intrinsic heat transfer means, the variation in temperature can bemonitored and regulated so as to comply with a relationship fortemperature variation as a function of time that is suitable forreducing internal tensions.

Once the desired temperature has been reached, the dies 1, 2 are movedtowards each other as shown in FIG. 3 so as to shape the thermoplasticmaterial that has been made malleable by the heat, and deform itplastically until a predetermined relative position has been reachedthat corresponds to the thickness desired for the lens.

Before and/or while the dies 1, 2 are being moved towards each other inorder to shape the material by plastic deformation, the suction channel25 and the vacuum pump (not shown) connected to the coupling 26 are usedto establish a relative vacuum in the sealed inside volume of the mold,as defined above.

Best results have been obtained with a relative vacuum of lower than0.25 bar, and preferably lying in the range 0 to 0.1 bar.

After the material has been fully shaped by moving the dies towards eachother to obtain predetermined compression (configuration of FIG. 3), thetrimming ring 30 is actuated to cut off the peripheral flash formed bysurplus material that has flowed outwards during compression and thatprojects sideways from the dies 1, 2. For this purpose, the trimmingring 30 co-operates with the lip 24 forming a margin of the annularrecess so as to act like a pincer (as shown), or in the alternative soas to act like a shear.

The dies 1, 2 and the sheath 3 are then cooled by means of theirintrinsic heat transfer means, down to an unmolding temperature. Forthis purpose, and in the example shown, the heat-conveying fluid iscooled by the external regulation source(s) S and is caused to circulatein the circuits 15, 16, 19, 20.

It is preferable for the unmolding temperature to be significantly belowthe vitreous transition temperature. By way of example, provision can bemade for the unmolding temperature to be less than half the vitreoustransition temperature. It should be observed that during this cooling,as during heating, the intrinsic heat transfer means enable temperaturevariation to be monitored as regulated to comply with a relationship fortemperature variation as a function of time which is suitable forreducing internal tensions.

Finally, it suffices to extract the finished lens L from the mold. Forthis purpose, the two portions 7, 8 of the sheath 3 are separated fromeach other as are the two dies 1, 2. Direct access is thus provided tothe gap situated between the dies 1, 2 and to the finished lens L whichrests on the bottom die 1. It then suffices to take hold of the lens Lby means of a handling tool such as tongs for manual use or associatedwith an automatic handling arm.

It is also necessary to extract the peripheral flash that was cut off bythe trimming ring 30 from the annular recess.

The mold described above can be used

-   -   either individually in the production of lenses in small        quantity or even one by one (or pair by pair), that is made to        order to satisfy a prescription order,    -   or in a mass production device including several molds of the        same type for the hot-forming of a corresponding number of        lenses at each cycle simultaneously.

Furthermore, starting from these main characteristics and steps of themold and method according to the present invention, several variationsor additions can be envisaged. These variations or additions,undermentioned can be implemented separately or in combination with eachother in order to directly obtain a finished or semi-finished coated, orat least treated, lens.

According to a first additional characteristic, the parison or preform Pcan advantageously be pre-coated with any single or multi layer coatingbefore being placed between the two dies.

According to a second additional characteristic, before heating the diesand the sheath, a thermoplastic film is disposed between the parison orpreform and at least one of the two dies. This thermoplastic film isthen applied on and self-welded to the corresponding face of the lens byso called hot-lamination during the shaping step, similarly to theprocess described in the document WO99/24243 which is incorporated byreference to the present specification. Advantageously, the film can becoated with any single or multi layer coating. Alternatively or incombination with a coating associated with the film, the film can alsobe photochromic and/or polarized to provide the lens with correspondingproperties. The film can also be tinted and/or colored.

According to a third additional characteristic, a single or multi layercoating can be disposed on at least one of the two dies before heatingthem. This coating is then hot-transferred on the corresponding face ofthe lens during the shaping step, similarly to the process described inthe document U.S. Pat. No. 5,458,820 which is incorporated by referenceto the present specification.

Broadly speaking, the invention is not limited to the particularembodiment described above, but on the contrary covers any variant usingequivalent means to reproduce the essential characteristics of theinvention. In particular, although the invention is described and shownwith a mold whose intrinsic heat transfer means are integrallyimplemented in the form of an internal circuit for circulating aheat-conveying fluid, it is equally possible, without going beyond theambit of the invention, to make such heat transfer means in any otherknown way, in particular concerning heating function by means ofelectrical resistances or by high frequency induction using coils, or byInfrared or Ultra-violet heating.

1. A mold for hot-forming a thermoplastic lens, the mold comprising twodies mounted in a sheath to slide along an axis of the sheath, and eachpresenting a transverse forming face facing the other, wherein the twodies and the sheath are provided with intrinsic and both-way heattransfer means, wherein said heat transfer means is adapted to athermoplastic material lens.
 2. A mold according to claim 1, in whicheach die comprises: a base containing at least a fraction of the heattransfer means of said die, and means for connecting them to a heattransfer source; and a removable insert fitted to the base and carryingthe forming face.
 3. A mold according to claim 2, in which the base ofeach die receives all of the heat transfer means of said die, the inserthaving no such means, being temperature regulated solely by heattransfer with the base.
 4. A mold according to claim 1, in which thesheath possesses an annular recess open to the inside and communicatingwith the interstitial gap defined by the forming faces of the two dies.5. A mold according to claim 4, in which the interstitial gap defined bythe forming faces of the two dies co-operates with the annular recess toform a sealed inside volume which is connected to the outside via atleast one suction channel passing through the sheath.
 6. A moldaccording to claim 5, in which the suction channel opens out into theannular recess.
 7. A mold according to claim 4, including a trimmingring slidably mounted in the sheath.
 8. A mold according to claim 7, inwhich the trimming ring co-operates with a lip bordering the annularrecess.
 9. A mold according to claim 4, in which the sheath has twoportions mounted to slide relative to each other along the same slidingaxis as the dies between firstly an open configuration giving directaccess to the interstitial gap defined by the forming faces of the twodies and secondly a closed configuration in which the two portions arein contact via junction faces in a transverse join plane.
 10. A moldaccording to claim 9, in which the annular recess of the sheath isimplemented in the form of a groove formed as a setback in the junctionface of at least one of the two portions of the sheath.
 11. A moldaccording to claim 10, in which the groove possesses draft taper.
 12. Amold according to claim 1, in which the heat transfer means comprise aninternal circuit for circulating a heat-conveying fluid.